A US military prosecutor at Guantanamo Bay has resigned over ethical disputes with his superiors, claiming they suppressed evidence that could help clear a young Afghan detainee of alleged war crimes.

The prosecutor, Lieutenant Colonel Darrel Vandeveld, described the disagreements in a statement supporting a defence plea to dismiss the charges against Mohammed Jawad.

"Potentially exculpatory evidence has not been provided," Lieutenant Colonel Vandeveld wrote, citing failure by the "prosecutors and officers of the court".

The disclosure triggered new attacks on the integrity of the US military tribunal system, which has faced accusations from other insiders of ethical breaches and political interference.

The current chief prosecutor, Colonel Lawrence Morris, denied his office withheld evidence and said there was no basis to Lieutenant Colonel Vandeveld's ethical qualms. He claimed the officer had told him he was leaving his post for "personal reasons".

"All you have is someone who is disappointed because his superiors didn't see the wisdom of his recommendations in a case," Colonel Morris said.

Mr Jawad, who was captured in Afghanistan when he was 16 or 17, is accused of throwing a grenade that wounded two US soldiers and their interpreter in December 2002. He faces a maximum life sentence at a military trial due to begin in December.

Lieutenant Colonel Vandeveld said prosecutors knew Mr Jawad might have been drugged before the attack and that the Afghan Interior Ministry had revealed two other men had confessed to the crime, according to Michael Berrigan, deputy chief defence counsel in Guantanamo. In his declaration, he said he wanted to offer Mr Jawad a plea deal that would allow him to receive rehabilitation during a short period of additional confinement. His superiors disagreed.

A tribunal official said Lieutenant Colonel Vandeveld would not comment.

Mr Jawad's defence lawyer, Major David Frakt, said he asked for Lieutenant Colonel Vandeveld to testify at a hearing yesterday, but he was refused permission to fly to Guantanamo.

A former Guantanamo prosecutor, Colonel Morris Davis, testified last month that a Pentagon official who oversaw the tribunals until last week, Brigadier General Thomas Hartmann, pushed for Mr Jawad to be prosecuted before others because details of the case would grip the US public and help build support for the process. "The guy who threw the grenade was always at the top of the list," Colonel Davis said.